A pn-junction type light-emitting diode (LED) is known as one example of a compound semiconductor light-emitting device. For example, a GaP-based LED is known, in which a GaP layer formed by epitaxial growth of a gallium phosphide (GaP) single crystal on a substrate is used as a light-emitting layer. A red range and orange to green range LED comprising an aluminum gallium arsenide mixed crystal (composition formula: AlXGaYAs (0≦X, Y≦1 and X+Y=1) or an aluminum gallium indium phosphide mixed crystal (composition formula: AlXGaYInZP (0≦X,Y,Z≦1, X+Y+Z=1) as a light-emitting layer is also known. A short wavelength (near ultraviolet, blue or green range) LED comprising a gallium nitride-based compound semiconductor layer such as gallium indium nitride (composition formula: GaαInβN (0≦α, β≦1 and α+β=1) as a light-emitting layer is also known.
In the above AlXGaYInZP-based LED, a conductive n-type or p-type light-emitting layer is formed on a substrate as a conductive p-type or n-type gallium arsenide (GaAs) single crystal. In a blue LED, a single crystal of electrically insulating sapphire (α-Al2O3 single crystal) is used as the substrate. In a short wavelength LED, silicon carbide (SiC) of a cubic crystal (3C crystal type) or hexagonal crystal (4H or 6H crystal type) is used as the substrate. On a semiconductor wafer in which a semiconductor layer is stacked on these substrates, for example, a first conductive transparent electrode and a second conductive electrode are provided to form a light-emitting device.
Particularly, in the case of a gallium nitride-based compound semiconductor light-emitting device, a gallium nitride-based compound semiconductor light-emitting device is formed by forming a layer on substrates made of various oxides or Group III-V compounds, including a sapphire single crystal, using a metal organic chemical vapor deposition (MOCVD) method or a molecular beam epitaxy (MBE) method.
The feature of the gallium nitride-based compound semiconductor light-emitting device is that it barely diffuses current in the longitudinal direction. Therefore, electric current is injected only into a semiconductor directly under an electrode, and light emitted in the light-emitting layer is not extracted to the outside since the light is blocked by the electrode. In the gallium nitride-based compound semiconductor light-emitting device, a transparent electrode is usually used as a positive electrode, and thus light is extracted through the positive electrode.
Regarding a transparent electrode, known conductive materials such as Ni/Au and ITO are used. For example, it has been recently proposed in Japanese Unexamined Patent Publication (Kokai) No. 2005-123501, that an oxide-based transparent electrode containing In2O3 or ZnO as the main component is used, since it is excellent in transparency. In the case of an ITO which is generally used as the transparent electrode, a conductive oxide film having low resistivity of 2×10−4 Ωcm or less can be obtained by doping In2O3 with 5 to 20% by mass of SnO2.
For example, it has been proposed in Japanese Unexamined Patent Publication (Kokai) No. 2000-196152, that the light extracting surface is subjected to uneven working so as to improve light extraction efficiency. An ITO having low resistance forms a fine crystal immediately after forming a film, and it is necessary to use an etching solution such as an aqueous ferric chloride (FeCl3) solution or hydrochloric acid (HCl) to subject ITO to uneven working. In wet etching using such a strong acid, etching takes place at a high rate, and thus it is difficult to control and burrs are easily generated at the edge portion of the ITO. Further, overetching easily occurs, resulting in poor yield.
The IZO conductive film described in Japanese Unexamined Patent Publication (Kokai) No. 08-217578 can be used to solve the above problems. The IZO film formed by a sputtering method is amorphous, and thus etching can be performed slowly without using the above strong acid. Therefore, burrs caused by etching and overetching do not easily occur. Furthermore, detailed processing for improvement of the output of a light-emitting device can be easily carried out.
However, the amorphous IZO film is inferior in transparency compared with the heat-treated ITO film, and therefore shows low output of the light-emitting device. Furthermore, there is a problem that the device has high drive voltage, since contact resistance with a p-type GaN layer is high, and the device is inferior in water resistance and chemical resistance, since it is amorphous, which results in a decrease in manufacturing yield after forming the ITO film and reliability of the device.